doc: document how to run tests

Dependencies

• [2] 3JXCTKEC Add markdown files for documentation

Change contents

• edit in doc/manual/src/hacking.md at line 29
  [2.7462]
  +

  + You can run Hydra's test suite with the following:

  +

  + [nix-shell]$ make check

  + [nix-shell]$ # to run as many tests as you have cores:

  + [nix-shell]$ make check YATH_JOB_COUNT=$NIX_BUILD_CORES

  + [nix-shell]$ # or run yath directly:

  + [nix-shell]$ yath test

  + [nix-shell]$ # to run as many tests as you have cores:

  + [nix-shell]$ yath test -j $NIX_BUILD_CORES

  +

  + When using `yath` instead of `make check`, ensure you have run `make`

  + in the root of the repository at least once.

  +

  + **Warning**: Currently, the tests can fail

  + if run with high parallelism [due to an issue in

  + `Test::PostgreSQL`](https://github.com/TJC/Test-postgresql/issues/40)

  + causing database ports to collide.